Samples of epithelium and cancers from human urinary bladders were obtained during cystoscopy or after cystectomy and examined by transmission electron microscopy. Cystectomy specimens proved superior with less disruption of epithelium, however both cystectomy and cystoscopy allowed adequate results under limited and controlled circumstances. The transitional epithelium of human urinary bladders, as in other mammals, had a protective layer of large superficial cells with an unique asymmetric trilaminar plasma membrane which lined the lumen and cytoplasmic vesicles of many shapes. Cells beneath the superficial layer lacked the asymmetric membrane and vesicles. Deeper cells were shaped irregularly; some were columnar, and many extended processes to the basement membrane. Pseudostratification of deep cells may relate to the flexibility of the bladder wall. Viruses were not observed in normal (atypical or cancerous) epithelial cells. Preoperative radiation produced inflammatory and degenerative changes in transitional epithelium with condensation and eventual lysis of cells. The effect of chemotherapy could not be judged. Carcinoma of the bladder was preceded, accompanied, and followed by epithelial atypia. The atypical cells appeared similar to those of tumors with simplification of cytoplasmic ultrastructural features and loss of surface specialization. In fact, traditional categories of cancer and precancerous epithelium were obscured by electron microscopy. BIBLIOGRAPHIC REFERENCES: Papillary (transitional) carcinoma in an ileal conduit. Banigo, O.G., Waisman, J., and Kaufman, J.J. J. Urol. 114:626-627, 1975. Urethral adenocarcinoma, Sacks, S.A., Waisman, J., Apfelbaum, H.B., Luke, P., and Goodwin, W.E. J. Urol. 113:50-55, 1975.